I apologize if this has been posted before. I've recently started playing T more some on ladder and wanted to know how to do this, but I searched and found no guide. After spending a little bit of time figuring it out, I thought I would share a neat little trick that will ensure you never miss a low SD/Rax wall.
Poorly crafted in MSPaint, you'll see an example of each iteration of a narrow ramp (backwater gulch, typhon peaks, delta quadrant and metalopolis, but this works on every narrow ramp).
The simple trick is to position your Depot/Rax such that it makes a 5-square seal around the concave green square at the bottom of your ramp.
With this in mind you'll note that each wall, although seemingly goofy, is impassable to scouting worker or any other ground unit.
Here's an example of each wall position on Shakuras:
While some of them look quite permeable, try and see for yourself!
I hope this helps anyone out there who has been frustrated about this in the past.
If you choose to low ramp wall with your first SD/Rax:
I highly recommend doing 10depot 11 rax instead of the standard 10 depot 12rax. If you plan to open standard OC, it is economically the same to cut the 12th SCV for 8 seconds in order to immediately drop the rax at 11 supply (assuming you go for standard OC timing, which would be at 14 vs 15 if you 11rax).
You can refer to this thread for the data: http://www.teamliquid.net/forum/viewmessage.php?topic_id=179108
But be wary of the possible perils of low ramp walling:
vs Michael vs Vasso